1. Field of the Invention
The present invention relates to a secondary battery and its method of manufacture, and more particularly, the present invention relates to the structure of an electrolyte injection-hole for injecting an electrolyte and an assembly that seals the electrolyte injection-hole and its method of manufacture.
2. Description of the Related Art
Recently, compact and light electronic devices such as cellular phones, notebook computers, camcorders and the like have been actively developed and produced. These portable electronic devices contain a battery pack to be operable without being plugged into an electrical outlet. The battery pack is provided with at least one battery therein so as to output a certain level of voltage to drive the portable electronic device. Battery packs have been developed to use rechargeable secondary batteries in consideration of economic views in recent years. Furthermore, a secondary battery has become a favorite choice for hybrid car batteries which require high density energy and high power.
Representative secondary batteries include nickel-cadmium Ni—Cd batteries, nickel-hydrogen Ni-MH batteries, lithium Li batteries, and lithium ion batteries, etc.
Especially, lithium secondary batteries have been widely utilized because their energy density per unit weight is high, and their operation voltage is three times higher than nickel-cadmium batteries or nickel-hydrogen batteries used as the power supply of a portable electronic device. In such a lithium secondary battery, active material of the positive electrode is made primarily from a lithium-based oxide, while active material of the negative electrode is formed of a carbon material. Generally, lithium secondary batteries can be classified into liquid electrolyte batteries and polymer electrolyte batteries based on the kind of electrolyte used. Accordingly, batteries using a liquid electrolyte are referred to as lithium ion batteries, while batteries using a polymer electrolyte are referred to as lithium polymer batteries. Furthermore, lithium secondary batteries are manufactured into various types, for example cylindrical types, prismatic types and pouch types.
Explaining the manufacturing process of general secondary batteries, first of all, an electrode assembly is manufactured by sequentially stacking and winding a positive electrode plate coated with the positive active material and including a positive electrode tap connected a collector of positive electrode, a negative electrode plate coated with the negative active material and including a negative electrode tap connected a collector of negative electrode, and a separator interposed between the positive electrode plate and the negative electrode. And, a pillar shape in an oval of electrode assembly is made by compression. After adhering the finishing tape to the outer surface of the electrode assembly, then inserting in the can to settle the insulating case in the upper portion of the electrode assembly. At the same time, uniting the cap plate, the insulating plate, and the terminal plate, the cap assembly is formed to unite the electrode terminal including the insulating gasket in the central hole configured with the cap plate, the insulating plate and the terminal plate. Thereafter, the electrolyte is injected to the electrolyte injection hole formed in the cap plate after sealing the can with the cap plate of the cap assembly. Then the secondary battery is completed by sealing the electrolyte injection hole. The method of sealing the electrolyte injection hole includes inserting by force a ball in the direction of the electrolyte injection hole after inserting the ball, having a greater diameter than that of the electrolyte injection hole into the electrolyte injection hole, an then welding the ball inserted with pressure and the area around the ball to seal up the electrolyte injection hole.
However, the problem of leaking electrolyte or electrolyte gas occurs because the electrolyte injection hole is not sealed closely in the period of inserting the ball with pressure. At this point, if the ball inserted into the electrolyte injection hole is welded, the excessive current flows in the leaking electrolyte, which enables the ball inserted with pressure and the area around the ball to melt for forming the crack. Further, for solving these problems there incorporated the coating process of resins to the ball inserted with pressure since welding, wherein the coated resins apt to drop by the exterior shock and the manufacturing cost increased due to the coating process of resins added.